1. Field of the Invention
The present invention relates to an optical receiver that installs an avalanche photodiode (hereinafter denoted as APD) with a controller for a bias voltage applied thereto in order to follow the optical input power provided to the APD.
2. Related Prior Art
The APD has been well known as a light-receiving device in an optical communication system. A system installing the APD provides a protection circuit to prevent an excess current from flowing in the APD by adjusting a bias voltage applied thereto when a large optical power is input into the APD. JP-A-10-308636, JP-A-08-186924, and JP-A-10-303820 have disclosed such protection circuits.
The first patent, JP-A-10-308636, has disclosed that the optical receiver monitors the current flowing in the APD, and the bias voltage applied to the APD is adjusted based on thus monitored current. But, once the bias voltage is decreased, the optical receiver is unable to detect the recovery of the optical input because the carrier multiplication factor of the APD strongly depends on the bias voltage. Accordingly, the bias voltage may not automatically recover its normal value.
The second patent, JP-A-08-186924, detects an anomaly of the bias voltage applied to the APD by a comparator and shuts off the bias voltage. This optical receiver does not install any monitoring function of the optical input power. The recovery of the bias voltage is necessary in manual.
The last patent, JP-A-10-303820, has disclosed that, by monitoring the optical input power with a PIN-PD prepared independent from the APD, the bias voltage to the APD is controlled based on the optical power monitored by the PIN-PD. However, such configuration to monitor the optical input power with the PIN-PD requires additional components such as an optical coupler and the PIN-PD, which makes the optical receiver complex and accordingly uncompetitive in cost.